Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communication between a base station and one or more subscriber units. The base station is typically a computer controlled set of switching transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units: The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a wireless cellular modem.
The most common type of antenna used to transmit and receive signals at a mobile subscriber unit is an omni-directional monopole antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto the antenna element at a specified frequency assigned to that subscriber unit. Forward link signals received by the antenna element at a specified frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency and use coding algorithms to detect signaling information intended for individual subscriber units on a per unit basis.
The transmitted signal sent from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of signals with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal on one direction versus detection of the same or a different signal coming from another direction.
Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. Typically, an antenna array with scanning capabilities consists of a number of antenna elements located on top of a ground plane. For the subscriber unit to satisfy portability requirements, the ground plane must be physically small. For example, in cellular communication applications, the ground plane is typically smaller than the wavelength of the transmitted and received signals. Because of the interaction between the small ground plane and the antenna elements, which are typically monopole elements, the peak strength of the beam formed by the array is elevated above the horizon, for example, by about 30xc2x0, even though the beam itself is directed along the horizon. Correspondingly the strength of the beam along the horizon is about 3 db less than the peak strength. Generally, the subscriber units are located at large distances from the base stations such that the angle of incidence between the subscriber unit and the base station is approximately zero. The ground plane would have to be significantly larger than the wavelength of the transmitted/received signals to be able to bring the peak beam down towards the horizon. For example, in an 800 Mhz system, the ground plane would have to be significantly larger than 14 inches in diameter, and in a PCS system operating at about 1900 Mhz, the ground plane would have to be significantly larger than about 6.5 inches in diameter. Ground planes with such large sizes would prohibit using the subscriber unit as a portable device. It is desirable, therefore, to direct the peak strength of the beam along the horizon with antenna elements mounted on a small ground plane so that the subscriber unit is mobile. Further, it is desirable to produce antenna elements with these beam directing features using low-cost mass production techniques.
The present invention greatly reduces problems encountered by the aforementioned prior art antenna systems. The present invention provides an inexpensive antenna for use with a mobile subscriber unit in a wireless same frequency network communications system, such as CDMA cellular communication networks. The antenna is isolated from the ground with a choke or narrow microstrip. The antenna is fabricated with printed circuit board (PCB) photo-etching techniques for precise control of the printed structure to mass produce antenna elements having repeatable features.
In one aspect of the invention, the dipole antenna includes a planar substrate made of dielectric material. A conductive planar element is layered on one side of the substrate, and a conductive planar ground patch is layered on the other side of the substrate. The conductive planar element is located in an upper region of the substrate, while the location of the planar ground patch is offset from the conductive planar element in a lower region of the substrate, that is, the conductive planar element is stacked above the conductive planar ground patch. A feed strip is connected to the conductive planar element, extends from the element to a bottom edge of the substrate, and terminates at a bottom feed point.
The conductive planar ground patch includes two portions. One portion extends from the midsection of the second portion to the bottom edge of the substrate and provides a connection point for coupling the conductive planar ground patch to a ground plane which is aligned orthonormally to the substrate.
Capacitive coupling between the conductive planar element and the conductive planar ground patch creates a junction which provides an upper dipole feed point in a mid- region of the substrate such that the conductive planar element acts as a first element of an unbalanced dipole antenna and the conductive planar ground patch acts as a second element of the unbalanced dipole antenna. The unbalanced dipole antenna forms a beam which may be positionally directed along a horizon that is substantially parallel to the ground plane.
Embodiments of this aspect can include one or more of the following features. The conductive planar element includes a base that is aligned parallel to a top edge of the substrate. The planar element also has a middle arm connected to a midsection of the base, and two outer arms connected to either end of the base. Each of the three arms are aligned perpendicularly to the base and extend towards the top edge of the substrate. The feed strip is connected to the midsection of the base and has an enlarged section. This size and location of this enlarged section can be varied to match the impedance of the dipole antenna with the feed impedance.
One portion of the conductive planar ground patch has a top strip aligned parallel to the bottom edge of the substrate. Located on either end of the strip is an arm which extends downward towards the bottom edge. The other portion of the conductive ground patch is a middle strip aligned perpendicularly to the bottom edge of the substrate. The downward extending outer arms can flare away from this middle strip to prevent coupling between the resonating outer arms and the middle strip which is connected to the ground plane. The lengths of these outer arms are approximately equal in length to a quarter wavelength of the transmitted and received signals.
The lengths of these outer arms as well as that of the arms of the conductive planar element can be varied to change the transmission frequency of the dipole antenna. If the lengths of the arms are approximately equal to one another, the dipole antenna transmits over a narrow bandwidth. For example, the dipole antenna is capable of operating with a bandwidth of about 10%. Alternatively, the lengths of the arms can be at different lengths to widen the bandwidth of the dipole antenna, for example, to a bandwidth of about 15%. Or the lengths can be varied so that the antenna operates at two or more frequencies.
The dielectric substrate can be made from, for example, common PCB substrate materials such as polystyrene or Teflon. The conductive planar element and the conductive planar ground patch are typically made from copper. There can be a layer of gold applied to the outer surface of the copper layers. Alternatively, there can be a layer of solder or a solder mask applied to the top of the copper layer.
In one embodiment of the invention, the conductive planar element is connected to a phase shifter. The phase shifter is independently adjustable to affect the phase of a respective signal transmitted from/to the dipole antenna. Alternatively, or additionally, the planar element can be connected to a delay line and/or a switch. Or the planar element can be connected to a lumped or variable impedance element, with or without the delay line and/or switch. The planar element is also connected to a transmission line which is used to transmit signals to and receive signals from the dipole antenna. Ideally, the peak strength of the directed beam rises no more than about 10xc2x0 above the horizon.